Device and method for electrochemically coating a workpiece

ABSTRACT

The invention relates to an apparatus ( 1, 101, 201 ) for electrochemically coating a workpiece ( 22 ), including a coating reservoir ( 2, 102, 202 ) with a charge opening ( 5 ) and a supply reservoir ( 3, 103, 203 ), wherein the two reservoirs are in communication through a passage opening ( 4, 104, 204 ). The apparatus has at least one electrode ( 11 ) and one counter electrode ( 14 ). In order to prevent contamination of a coating liquid ( 20, 120, 220 ) during an electrochemical deposition process, the apparatus can be switched between a coating position and a rest position by rotating the supply and the coating reservoirs about at least one common axis (A, A′, A″) so that the center of gravity of the volume of the coating reservoir ( 2, 102, 202 ) in the coating position is lower in relation to the center of gravity of the volume of the supply reservoir ( 3, 103, 203 ) than in the rest position. The invention further relates to a method for coating workpieces.

BACKGROUND OF THE INVENTION

The invention relates to a device and a method for electrochemically coating a workpiece.

A great number of methods for electrochemically coating workpieces are known. Primarily, methods for electroplating on the one hand and methods for electrochemical dip painting on the other, are to be distinguished. These methods have in common that the workpiece is at least partially introduced into a bath of a coating liquid.

In the electrochemical dip process, an electrically conductive dip paint is the coating liquid. The dip paint frequently contains metal particles. For coating, a voltage is applied between the workpiece and the counter electrode. By these means, binders dissolved in the bath and/or metal particles precipitate on the surface of the workpiece, resulting in a closed, adhering paint coat, or a metal-containing coating. Depending on whether the workpiece is connected as an anode or a cathode, a distinction is made between anodic dip painting and cathodic dip painting.

In electroplating, the coating bath contains ions of one or more elements with which the workpiece is to be coated. Typically, the ions are contained in a solvent in the coating liquid, it also being possible, however, to use a melt of a salt containing the relevant ions. For coating, a voltage is applied to the workpiece, which leads to the ions present in the coating liquid being electrolytically discharged on the surface of the workpiece. This results in the deposition of a layer on the surface. The ions are often cations and the workpiece is an anode, on which the cations are reduced.

The solvent in which the ions are dissolved is water in many cases. There are applications, however, in which water is not really suitable, if not completely unsuitable. One reason for this may be that water itself can basically be subject to electrolysis. For example, certain metals cannot be deposited from an aqueous solution since the water is electrolyzed instead.

SUMMARY OF THE INVENTION

The use of alternative solvents instead of water is more expensive on the one hand, and on the other, these solvents are often also susceptible to contamination. Contamination by air moisture, in particular, can lead to a substantial deterioration of the coating process. Some of these solvents react with water and become useless after contamination therewith. This is particularly true for the so-called ionic liquids, i.e. salts, having a melting point of below 100° C. However, the use of ionic liquid in particular is the preferred choice in the deposition of certain metals, such as aluminum. In the context of an industrial coating method, therefore the particular sensitivity of such solvents must be taken into consideration.

It is therefore the object of the invention to suggest measures to prevent contamination of a coating liquid in the context of an electrochemical deposition process.

The method is achieved by an apparatus and a method as disclosed herein.

The apparatus according to the present invention for electrochemically coating a workpiece comprises a coating reservoir for receiving the workpiece, and a supply reservoir for a coating liquid. The above-mentioned reservoirs must have sufficient stability to be able to receive the workpiece and the coating liquid respectively. Metals, in particular stainless steel, are suitable materials for the construction. Non-metallic materials such as ceramics or glass are also suitable. When selecting the material, it must be taken into consideration, that those parts of the reservoir that can come into contact with the coating liquid, in particular, should be insensitive to the latter. If, for example, the coating liquid were to dissolve the material of the reservoir—even if only very slightly—it would substantially wear out the reservoir, while contaminating the coating liquid at the same time. This must be taken into consideration, in particular, if ionic liquids are used, since they are often excellent solvents. To prevent the reservoir being corroded by the coating liquid, each reservoir can be wholly or partially provided with a protective coating.

According to the present invention, the supply reservoir communicates with the coating reservoir by at least one passage opening for the coating liquid. The coating liquid can pass through the passage opening from the supply reservoir into the coating reservoir, and vice versa. The passage opening should be relatively small in relation to the dimensions of the coating and supply reservoirs, to minimize the introduction of gases and moisture dissolved therein, from the coating reservoir into the supply reservoir. The cross section of the passage opening should be sufficiently large, however, to ensure unobstructed passage of the coating liquid. By these means, the supply reservoir, in particular in its rest position (see below), forms a largely closed system even without a closure mechanism for the passage opening.

It is conceivable for the passage opening to be configured as a tube or a hose, to establish communication between the coating reservoir and the supply reservoir. Alternatively, the two reservoirs can be directly adjacent, wherein, in the interface area, the passage opening establishes communication. The latter option leads to a smaller inner surface area of the system and is therefore preferable, since contamination sources for the coating liquid are thereby minimized. The importance of the surface will be further explained in the following.

The passage opening can comprise a mesh, screen or filter. By these means, it can be prevented on the one hand that a small workpiece passes into the supply reservoir, on the other hand, certain contaminants, introduced together with the workpiece, can be kept out of the supply reservoir.

It goes without saying that the apparatus must comprise at least one charge opening through which the workpiece is directly or indirectly introduced into the coating reservoir. The charge opening is preferably formed on the coating reservoir itself so that direct introduction is possible. The term “charge opening”, in the present context, has a wide range of meanings. Structures are also included in which, for example, about half of the coating reservoir is removed to introduce the workpiece and is replaced again after introduction. Generally, the opening is configured in such a way that it allows the introduction of the workpiece and, as the case may be, in the case of small bulk parts, of a basket or the like containing the workpieces.

It is also necessary as known from the state of the art of electrochemical coating methods, for the apparatus to comprise at least two electrodes. At least one electrode for contacting the workpiece is for applying an electric potential to the latter, while at least one counter electrode is in contact with the coating liquid during coating. A plurality of electrodes for contacting and/or a plurality of counter electrodes could also be present. The former can be advantageous, in particular, if a plurality of workpieces is present, since occasionally simultaneous contacting of a plurality of workpieces may thus be advantageously implemented.

According to the present invention, the apparatus can be switched between a coating position and a rest position by tilting the coating reservoir and the supply reservoir about at least one common axis so that the center of gravity of the volume of the coating reservoir, in the coating position, is lower in relation to the center of gravity of the volume of the supply reservoir than in the rest position.

Basically, the switching process according to the present invention between the coating position and the rest position may be implemented by tilting the coating reservoir about exactly one axis. It may also be conceivable, however, for the tilting to be performed about more than one axis. In the physical sense, whereby all possible positional changes of a body can be classified as rotations, translations or a combination thereof, it is also conceivable to combine the tilting movement, i.e. the rotation, with a translation. This means that the coating reservoir and the supply reservoir could also be traversed, for example, along a path with an increasing gradient, which corresponds to a motion combined of rotation and translation.

Preferably, the coating reservoir and the supply reservoir are rigidly connected with each other. A rigid connection leads to a reduction of moving parts, seals and the like. Moreover, it allows the apparatus to be generally designed in a more compact form, which leads to a smaller surface area.

The invention is based on the idea that surfaces and moving parts are a primary source of contamination of the coating liquid, in particular by moisture. Moisture can adhere to surfaces and is very hard to completely remove from them. Similarly, the coating liquid can adhere to surfaces and thus presents a large surface of attack for the surrounding atmosphere. Moving parts necessarily increase the overall surface area of an apparatus. Moreover, moisture can penetrate between moving parts. Therefore, if pumps or similar apparatuses are used, it is almost impossible to keep the interior of a coating apparatus free of moisture. In view of the above, the apparatus according to the present invention is configured in such a manner that pumps, hoses or the like can be dispensed with. Rather, the coating reservoir and the supply reservoir together with the passage opening form a comparatively compact system having the smallest possible interior surface area. There is thus only little surface area to which moisture can adhere. Also, the interior of the system comprising the supply reservoir and the coating reservoir is sealed with respect to the outside, apart from the at least one charge opening, which is necessary for the introduction and removal of workpieces.

Additionally, certain ionic liquids, such as alkylated imidazolium chlorides, when in contact with air moisture, form hydrogen chloride, which is highly aggressive and would directly corrode sensitive parts, such as seals. Complete shielding against moisture is almost impossible in the industrial process. It is thus a further advantage of the apparatus according to the present invention, which largely dispenses with moving parts, that any formation of hydrogen chloride can cause little or no damage.

It is also advantageous that the liquid, which is received in the supply reservoir in the rest position, there only communicates with the coating reservoir by means of the relatively small passage opening, while the coating reservoir in turn is in contact with the ambience, at least during the introduction and removal of workpieces. By these means the coating liquid is largely protected against contamination.

Contamination of a coating liquid, in particular by air moisture, can thus be minimized by the apparatus according to the present invention. By these means, undesirable reactions of the coating liquid can be avoided and their unlimited use can be ensured for a long period of time. This means that high-quality coatings are assured and the cost for replacement or reprocessing of the coating liquid is reduced.

There are also other advantages. The exclusion of moving parts results in a reduction of wear and tear. As already explained, this applies, in particular, if aggressive substances could be released, which corrode seals and similar parts. The apparatus needs to be maintained less frequently and can be in uninterrupted operation for longer periods of time. Cost for replacement parts, lubricants and the like is also eliminated.

Various arrangements of the reservoirs are conceivable for the apparatus according to the present invention, wherein here, for simplicity, the positions of the reservoirs will be referred to instead of the centers of gravity of their volumes. If the two reservoirs are on the same side of the rotation axis with respect to a vertical plane extending through the rotation axis (vertical in the sense of the direction defined by gravity), one reservoir being in a position further to the outside, this reservoir is shifted on rotation, which causes a rise on this side of the rotation axis, to a raised position with respect to the other.

If the two reservoirs are on the same side of the rotation axis at equal distances from the rotation axis, but one on top of the other, the initially lower reservoir is raised with respect to the initially top reservoir during a rotation which causes a rise on this side, until the former comes to lie side by side with the latter at the apex of the rotation. If the rotation is continued to the other side of the rotation axis, the initially lower reservoir comes to lie above the initially top reservoir. Thus the two reservoirs are essentially reversed in a manner similar to an hourglass.

If the coating reservoir and the supply reservoir are on opposite sides of a vertical plane extending through the rotation axis, it goes without saying that the reservoir on its side where the rotation causes a rise is shifted to a raised position with respect to the other. Such an arrangement in which the rotation axis is between the reservoirs has the advantage that the torques caused by the weight of the two reservoirs at least partially cancel each other out so that they are essentially “balanced”. When the reservoirs are positioned on opposite sides of the axis, only a small retaining moment or counterweight is necessary to prevent uncontrolled tilting. If one reservoir is substantially heavier than the other it is thus preferred for the heavier reservoir to be positioned closer to the rotation axis, or for the rotation axis to be directly adjacent to it.

The coating reservoir and the supply reservoir are always arranged thus that a rotation causes the one to be raised or lowered with respect to the other and vice versa. If the passage opening is suitably arranged, which can easily be determined by the person skilled in the art, coating liquid thus always flows from the reservoir that is raised to the one that is lowered. In most embodiments of the apparatus it should be recommendable to arrange the passage opening at the lowest possible position.

In the context of the present invention, “flow” means that the coating liquid only moves under the influence of gravity after switching the apparatus over, i.e. that there is no need for the use of pumps or other apparatuses for conveying. The coating liquid is thus essentially “poured” from one reservoir into the other. Such a flow can easily be implemented by configuring the lower part of the side wall of the supply reservoir in the coating position (in a way, the “bottom”) with a profile descending toward the passage opening, while the lower part of the side wall of the coating reservoir in the rest position, is configured with a profile descending toward the passage opening.

To ensure that the coating liquid flows to and fro as completely as possible, at least the inner surfaces of the coating reservoir and the supply reservoir are of a material which is hardly or not at all wetted by the coating liquid. By these means, it is avoided, in particular, that small residual volumes in the form of droplets or liquid films adhere to the surface of the reservoir. These residual volumes, which present a particularly large surface area to the surrounding atmosphere, are particularly susceptible to contamination. By using a suitable surface material, such as in the form of a coating, this can be effectively avoided.

In a preferred embodiment, the apparatus comprises an opening for gas exchange between the coating reservoir and the supply reservoir. Such an opening is advantageous since it enables pressure equalization between the two reservoirs even if the passage opening is completely filled with coating liquid. The flow of the coating liquid can thus be substantially facilitated, in particular, when the passage opening has a very small cross section, or if a very rapid liquid exchange is to be carried out between the two reservoirs. Since gases are always above the coating liquid within the reservoir, the opening for gas exchange is arranged above the passage opening for the coating liquid. The opening for gas exchange can be configured as a pipe or tube, or it is in an area where the coating reservoir and the supply reservoir are almost directly adjacent to each other and are practically in direct communication with each other through the opening.

Even though, in principle, it is possible to receive a workpiece in the coating reservoir during the coating process in a “loose” state, a receiving device for the workpiece is advantageously arranged in the coating reservoir. Such a receiving device is for supporting the workpiece during the coating process and—at least within certain limits—to secure it against positional shifts. The receiving device can be configured, for example, as a rack, a basket or the like. It is also possible for the receiving device to not directly receive the workpiece or workpieces, but, for example, a basket, in turn filled with small bulk parts, to be coated. As known from the state of the art, in this case the entire basket can be introduced into the coating reservoir and removed afterwards.

Preferably, the receiving device is positioned within the coating reservoir in such a manner that, in combination with a suitable filling level of the apparatus, it is ensured that a workpiece present in the receiving device is above the level of the coating liquid in the rest position. If the structure and the filling condition are chosen such that, as the apparatus is switched from the coating position into the rest position, all coating liquid flows into the supply reservoir, the receiving device can basically have any desired position within the coating reservoir. It must be noted, however, that, in the coating position, a workpiece present in the receiving device, must be below the level of the coating liquid—at least partially—for coating to be possible. However, the choice of a suitable structure and the associated filling condition is a standard task for the person skilled in the art.

In a preferred embodiment of the invention, the receiving device arranged within the coating reservoir is rotatably supported. According to a typical design option, such a receiving device is fixed on a shaft, which in turn is coupled to a motor or a motor-transmission unit. To avoid unnecessary stressing of the bearings during a rotation of the receiving device, a structure of the receiving device which is axis-symmetrical with respect to the rotation axis is advantageous.

By ensuring rotatability, in combination with a suitable motor-transmission unit, two functions can be fulfilled.

On the one hand, slow rotation can cause the workpiece to be tumbled during the coating process. This is important, in particular, if small bulk parts are to be coated. There are numerous contact points between them which would not be coated, or only in an incomplete manner, if the workpieces were resting. As the workpieces are moved, the contact points constantly change between them and the surface can be uniformly coated. To ensure efficient tumbling of the workpieces, it is suitable to choose a speed at which there are no strong centrifugal forces. It is thus preferred for the receiving device to be rotatable at a speed of at most 100 rpm in the coating position. As is well known to the person skilled in the art, it is advantageous if the rotation axis of the receiving device deviates from the vertical by at least 5°, preferably at least 20°, since tumbling will then be better promoted by the force of gravity.

On the other hand, the receiving device, rapidly rotated, can serve as a centrifuge for a workpiece present therein, so that coating liquid still adhering after coating can be spun off. For this purpose, preferably, the receiving device is rotated at a speed of at least 200 rpm when the apparatus is in the rest position. In this position, the coating liquid has at least partially drained from the coating reservoir into the supply reservoir, and the workpiece, as described above is preferably above the level of the coating liquid. The receiving device can thus be used for spinning-off like centrifuges, which are known from the state of the art. Within the above described method, the spinning is between step b) and step d), i.e. spinning, as the case may be, can be as early as during the switch-over into the rest position, which is during step c). This may be advantageous to save time, on the other hand it is obvious that spinning should only start after the coating liquid has drained to a point where the workpiece is above the liquid level.

It is possible to implement the tumbling function without the centrifuge function and vice versa. To efficiently carry out the method, wherein as many method steps as possible are implemented within one apparatus, it should be seen as advantageous to combine the two functions.

In view of efficiently using the available coating liquid, during the coating process, the largest possible proportion of the liquid should be in the coating reservoir, i.e. as little liquid as possible should remain in the supply reservoir. In particular, usually, in the coating position, the area of the receiving device—if present—is usually at least partially below the level of the coating liquid. It is thus preferred, in the coating position, for at most 50% of the volume of the supply reservoir to be below the passage opening. Particularly preferably, the entire volume of the supply reservoir is above the passage opening in the coating position.

Similarly, in the rest position, the coating reservoir should be as free of coating liquid as possible. For this reason, the apparatus according to the present invention has been further developed so that, in the rest position, a partial volume of the coating reservoir, which corresponds to at most 50% of the volume of the supply reservoir, is below the passage opening. Particularly preferably, in the rest position, the entire volume of the coating reservoir is above the passage opening.

The coating chamber can basically be exposed to a contamination risk by ambient air and thus associated moisture. Therefore, it is advisable that as little coating liquid as possible remains in the coating reservoir in the rest position, e.g. during introduction and removal of the workpiece, because, at least in this phase, gas exchange is possible between the ambience and the interior of the coating reservoir via the charge opening. Furthermore, a higher residual level of coating liquid in the coating reservoir is disadvantageous in the rest position, because in this position, usually, liquid still adhering to the workpiece is drained or the like. Therefore, care must be taken that no portion of the workpiece remains immersed in liquid remaining in the coating reservoir. This means, if a receiving device is present, in the rest position, it should be above the liquid level. This can be achieved all the more easily the lower the remaining level of residual liquid.

To make use of the supply reservoir as efficient as possible, in particular to ensure complete draining of the coating liquid into it, it is additionally provided that, in the rest position, the entire volume of the supply reservoir is below the passage opening. By these means, if appropriate filling is provided, which will be discussed below, it can be ensured that the entire supply reservoir is filled with coating liquid in the rest position.

In a further development of the invention, the coating reservoir has a closable charge opening to introduce the workpiece. To avoid contamination of the coating liquid, the charge opening is preferably closable in an air-tight manner. It is also conceivable to configure a lock chamber at the charge opening which is closable in an air-tight manner on both sides and thus completely prevents gas exchange between the ambience and the coating reservoir.

To avoid any contact between the coating liquid and the charge opening, the apparatus is preferably configured in such a way that, in the coating position, a partial volume of the coating reservoir, which is at least equal to the volume of the supply reservoir, is below the charge opening. By these means it is ensured that even if all coating liquid has flown from the supply reservoir into the coating reservoir, the charge opening is above the level of the coating liquid. Thus, on the one hand, the charge opening is protected against damaging influences, i.e. for the construction of seals or the like, it is not necessary to use materials which are insensitive to the coating liquid. Finally, it can be excluded, for example during opening of a closure of the charge opening, that coating liquid is inadvertently discharged.

Often the electrochemical coating process is crucially influenced by the temperature at which it is performed. Some ionic liquids, in particular, have shown sensitivity to temperature variations which, in the worst case, can lead to the liquid becoming completely useless. Properties such as viscosity and conductivity, which are crucial for the electrochemical coating process, are generally temperature-dependent. To ensure tempering of the coating liquid, the apparatus, in a further development of the present invention, comprises means for heating and/or means for cooling the coating liquid. Which means are necessary for this purpose also depends on the coating liquid used. If its optimum working temperature is between 70° C. and 80° C. and the apparatus is in an ambience at room temperature, means for cooling may well be dispensed with. If the optimum working temperature is between 15° C. and 20° C., however, means for heating are usually dispensable. At this it must be taken into consideration that heat is constantly introduced into the liquid during coating, since the coating current is subject to resistance losses within the liquid.

In any case, both means for cooling and means for heating can be of various kinds. Electric heating coils integrated in the reservoir side wall or extending into the reservoir interior are conceivable. Heat exchangers of the coil or lamella type, through which a warm or cool liquid flows, may also be used. Finally, it is also possible to directly pass warm or cool inert gas through the coating liquid. Infrared radiation may of course also be used for heating. If the passage opening is formed as a line, i.e. it has a certain longitudinal extension, means for heating or cooling could be arranged at its side wall so that the passage opening has the function of an instant heater or instant cooler.

Practically all coating liquids, in particular dip paints but also liquids for electroplating, show a certain sedimentation behavior. While the coating liquid is somewhat mixed as it flows from one reservoir into the other, it is advantageous for the apparatus to also comprise means for mixing the coating liquid. They may be configured in such a manner that they act on the liquid in the coating reservoir and/or in the supply reservoir. Such means can comprise a mechanical stirrer driven by means of a motor and a shaft. Magnetic stirrers are also advantageous since they do not require an additional opening in a reservoir side wall for mechanical coupling. Mixing by means of ultrasonic waves is particularly advantageous since it does not require any additional parts within each reservoir. All means for mixing can be operated either intermittently or continuously.

It is conceivable within the scope of the present invention for part of the side wall of the coating reservoir, the receiving device or parts of the same to be formed as an electrode for contacting the workpiece. In this connection it must be ensured that every workpiece is in direct or indirect (via other workpieces) electric contact to the electrode in question during the entire coating process (with the exception of any short interruptions). In a further development of the invention, however, at least one electrode is traversable into the coating reservoir to contact the workpiece. Such traversal can be carried out by a transmission, pneumatically, or hydraulically, the latter being preferred. At this the at least one counter electrode can either be fixedly installed or may also be traversable. In any case it must be ensured that the counter electrode is immersed in the coating liquid during the coating process.

It is particularly advantageous for at least one electrode to be flexibly supported for contacting the workpiece. Such support enables the electrode to follow any movements of moving workpieces and is as such in constant contact. The flexibility can be achieved by a cable or a type of hose. Typically, the electrode can be suspended from above which enables it to hold contact with the workpieces essentially due to the force of gravity.

The apparatus according to the present invention can be used to carry out a coating method. For this purpose the coating reservoir and/or the supply reservoir are at least partially filled with a coating liquid in advance. In the coating method according to the present invention the following steps are performed in succession:

-   a) introducing the workpiece into the coating reservoir, -   b) electrochemically depositing a coating on the workpiece while the     apparatus is in the coating position, -   c) switching the apparatus into the rest position, and -   d) removing the workpiece from the coating reservoir.

Optionally, the apparatus may be switched into the coating position prior to or after introducing the workpiece into the coating reservoir.

In a further development of the method, an apparatus is used that comprises a receiving device as well as motor means for rotating the receiving device, wherein the receiving device is rotated by the motor means at most at 100 rpm during the deposition of the metal layer.

To spin off coating liquid from the workpiece already within the coating reservoir, it is advantageous for the method to use an apparatus comprising a receiving device and motor means for rotating the receiving device, and for the receiving device to be rotated by the motor means at at least 250 rpm between step b) and step d).

In a preferred variant of the above explained coating method, an apparatus is used comprising at least one electrode traversable into the coating reservoir for contacting the workpiece, and the electrode is traversed into the coating reservoir prior to the deposition process, that is prior to step b), so that it contacts the workpiece at least temporarily during step b). After completing the deposition process, the electrode is traversed back out of the coating reservoir. Preferably the electrode is traversed into the coating reservoir after the introduction of the workpiece, and is traversed back out of the coating reservoir before the workpiece is removed. The electrode can be traversed back out of the coating reservoir before, during or after switching the apparatus into the rest position.

To ensure optimum protection of the coating liquid against contamination in the method, it is advantageous for a volume of the coating liquid used to be between 80% and 110%, preferably between 95% and 105%, particularly preferably between 99% and 101% of the volume of the supply reservoir. Little or no liquid thus remains in the coating reservoir in the rest position on the one hand, and on the other, the supply reservoir is filled with coating liquid as completely as possible. In this case, the only portion of the surface of the liquid in contact with gas from the coating reservoir is that which is present at the passage opening and at the opening for gas exchange, if any. This means that any potential contamination can only happen at these relatively small surface areas and can thus be largely avoided.

Preferably, in this method, an ionic liquid is used as a coating liquid, comprising ions of at least one element. These ions can either be part of the ionic liquid or can be present in the latter in a dissolved state. Elements useful for this purpose are metals and semiconductors, in particular, but non-metals are also explicitly included.

BRIEF DESCRIPTION OF THE DRAWINGS

The deposition of aluminum from an ionic liquid such as 1-ethyl-3-methylimidazolium chloride, in which an aluminum salt, such as aluminum chloride, is dissolved, can be carried out with the apparatus. Aluminum alloys can also be deposited. Alloys of aluminum with at least one of the elements from the group comprising silicon, iron, copper, manganese, magnesium, chromium, nickel, zinc, lead, and titanium are preferred.

Details of the invention will be explained in the following with reference to the drawings, wherein:

FIG. 1 a is a schematic representation of a first embodiment of an apparatus of the present invention in the rest position prior to charging;

FIG. 1 b is a schematic representation of the apparatus of FIG. 1 a in the rest position after charging;

FIG. 1 c is a schematic representation of the apparatus of FIG. 1 a in the coating position;

FIG. 2 a is a schematic representation of a second embodiment of an apparatus according to the present invention in the rest position;

FIG. 2 b is a schematic representation of the apparatus of FIG. 2 a in the coating position;

FIG. 3 a is a schematic representation of a third embodiment of an apparatus according to the present invention in the rest position; and

FIG. 3 b is a schematic representation of the apparatus according to FIG. 3 a in the coating position.

DETAILED DESCRIPTION

FIGS. 1 a to 1 c show a coating apparatus 1 according to the present invention in the rest position prior to (FIG. 1 a) and after charging (FIG. 1 b), and during the coating process (FIG. 1 c). Coating apparatus 1 comprises a coating reservoir 2 and a supply reservoir 3 in which a coating liquid 20 is present. Coating liquid 20 is a melt of 1-ethyl-3-methylimidazolium chloride in which aluminum chloride is dissolved. Coating liquid 20 tends to form hydrogen chloride when in contact with moisture, which is limited by low levels of air moisture in the dry chamber. The formation of CH gas is suppressed by constantly flushing coating reservoir 2 with dried air (relative moisture of the dried air at less than 0.1%).

Reservoirs 2, 3 are in communication with each other by a passage opening 4 for coating liquid 20. Additionally, a second communication is created by an opening 15 for gas exchange arranged above passage opening 4. Apparatus 1 is switchable between the rest position shown in FIG. 1 a and a coating position shown in FIG. 1c by tilting reservoirs 2, 3 about a common rotation axis A. Rotation axis A extends normal to the plane of the drawing. First axis A is provided by a shaft (not shown), by means of which reservoirs 2, 3 are coupled to a motor-transmission unit (also not shown) mounted on a base 16. Switching or tilting of apparatus 1 is implemented by means of the motor-transmission unit. In the apparatus 1 shown here, both reservoirs 2, 3 are integrally formed of a ceramic material or of glass, which is insensitive to hydrogen chloride.

In the rest position (FIGS. 1 a, 1 b) the entire volume of supply reservoir 3 is below passage opening 4, and the entire volume of coating reservoir 2 is above passage opening 4. The volume of coating liquid 20 is identical to the one of supply reservoir 3 so that, in the rest position, it is completely filled. Thus only a small portion of coating liquid 20 adjacent to passage opening 4 and to opening 15 for gas exchange is in contact with the air in coating reservoir 2. The—already low—level of moisture in the air present in coating reservoir 2 is thus only able to contact a very small surface of coating liquid 20 which further minimizes the formation of hydrogen chloride.

In FIG. 1 a, coating reservoir 2 has a charge opening 5, which is closable by a lid 6. Lid 6 is opened to introduce a basket 21. At the outside of coating reservoir 2, a pivotable and traversable support 12 of a contact electrode 11 is mounted, which can be traversed into coating reservoir 2 through an electrode opening 17 in lid 6. Support 12 also has a flexible portion 13. An aluminum electrode 14 is present within coating reservoir 2, acting as a counter electrode for contact electrode 11.

A receiving device 7 for a basket 21 containing workpieces 22 is arranged within coating reservoir 2. Receiving device 7 can be caused to rotate by means of a shaft 8 extending through the wall of coating reservoir 2. Receiving device 7, in the present case, is configured as a vat with an apertured side wall so that the basket 21 can be inserted in it and secured against larger positional changes while liquid 20 can pass through the side wall of receiving device 7 to basket 21.

Shaft 8 is drivable by means of a transmission 9 arranged at coating reservoir 2. The transmission in turn is driven by a motor 10. By these means, receiving device 7 can be rotated about a rotation axis B approximately identical to the symmetry axis of receiving device 7 and vertical in the rest position—in relation to the direction defined by gravity.

In the rest position, the centers of gravity of the volumes of the two containers 2, 3 are on different sides of a vertical plane extending through first axis A, wherein first axis A, however, extends through coating reservoir 2. The reason for this is that both support 12 with contact electrode 11 and receiving device 7, transmission 9 and motor 10 are coupled to coating reservoir 2, resulting in the effective center of gravity of mass of the system being on the side of coating reservoir 2. By arranging first axis A in the present way it is achieved that the torques caused by the weight of the components on either side at least partially cancel each other out so that, in a way, they are essentially “balanced”.

The functioning of coating apparatus 1 shown in FIGS. 1 a to 1 c will be explained in the following in an exemplary manner. 20 kg steel screws 22 are to be coated with aluminum. Coating should be carried out at room temperature. Screws 22 are prepared for coating by sand blasting first of all and subsequently degreased in a basket 21 in a cleaning solution comprising water, in which, in 1 liter water, 9 g potassium phosphate and 27 g potassium hydroxide have been dissolved, at 85° C. After a soaking time of 5 min, basket 21 is lifted out of the bath (not shown here). Basket 21 containing screws 22 is rinsed with tap water and subsequently spun dried. Then screws 22 are further dried by means of a hot air flow.

Dried screws 22 in basket 21 are transferred to coating apparatus 1. Basket 21 is inserted into the vat of receiving device 7, whereby it is secured against larger displacements. After closing lid 6, contact electrode 11 is traversed into coating reservoir 2 by means of support 12. This state is shown in FIG. 1 b.

Coating apparatus 1 is then switched into the coating position. In this position, the center of gravity of the volume of coating reservoir 2—in relation to the vertical plane extending through first axis A—is on that side of axis A on which the rotation results in a lowering motion, while the center of gravity of supply reservoir 3 is on that side on which the rotation causes a lifting motion. By these means, the center of gravity of the volume of supply reservoir 3 is shifted upwards with respect to that of coating reservoir 2. Coating liquid 20 flows through passage opening 4 into coating reservoir 2, while through opening 15 for gas exchange it is ensured that, even if passage opening 4 is completely filled with coating liquid 20, the pressures can be equalized.

As can be seen in FIG. 1 c, which shows coating apparatus 1 in the coating position, the profile of supply reservoir 3 is configured in such a manner that it slopes downward toward passage opening 4 in this position, which promotes draining. In the coating position, the entire volume of supply reservoir 3 is above passage opening 4. Screws 22 are washed with coating liquid 20, aluminum electrode 14 dips into coating liquid 20. At this flexible contact electrode 11 is loosely placed on top of screws 22 so that each screw 22 is directly or indirectly in electric contact with it.

For coating, receiving device 7 with basket 21 is caused to slowly rotate at 20 rpm while a voltage is applied between aluminum electrode 14 and contact electrode 11 so that aluminum electrode 14 functions as an anode. The voltage is adjusted such that a deposition current having a mean current density of 10 A/dm² results. Screws 22 which, by being loosely contacted with contact electrode 11 have the same electric potential as the latter, are coated by the deposition of aluminum from coating liquid 20, while aluminum ions are continuously detached from anode 14 by oxidation so that the aluminum concentration remains constant within coating liquid 20.

After a coating period of 5 min, an Al layer with a thickness of about 10 μm has been deposited on screws 22, the voltage is switched off, receiving device 7 is stopped and apparatus 1 is switched back into the rest position, whereafter the entire volume of supply reservoir 3 returns to below the level of passage opening 4 (cf. FIG. 1 b). The best part of liquid 20 thus drains from coating reservoir 2 into supply reservoir 3. However, liquid residue still adheres to the screws 22. To spin this off, receiving device 7 is caused to rotate at a fast 300 rpm. After three runs of spin drying, each lasting 20 seconds with a 10 second pause between them, screws 22 are almost free of coating liquid 20.

After spin drying, contact electrode 11 is backed out of coating reservoir 2. Lid 6 of charge opening 5 is opened and basket 21 is lifted out of coating reservoir 2. The coating method according to the present invention is thus completed.

The finished, coated screws 22 are subjected to an electroplating process in order to passivate the surface of the aluminum coating.

FIG. 2 a and FIG. 2 b, in a heavily schematic representation, show a further embodiment of an apparatus 101 for electrochemical coating. Herein, a coating reservoir 102 with a charge opening 105 and a supply reservoir 103 is on one and the same side of a common rotation axis A′, wherein, however, supply reservoir 103 is positioned further to the outside. In the rest position shown in FIG. 2 a, supply reservoir 103 is below a passage opening 104 and coating liquid 120 is received within it. After switching into coating position 101, the center of gravity of the volume of supply reservoir 103 is shifted upwards in relation to that of coating reservoir 102, which causes coating liquid 120 to flow into coating reservoir 102. To compensate for the torque caused by reservoirs 102, 103, these are coupled to a counterweight 118, which is positioned on the other side of rotation axis A′.

FIG. 3 a and FIG. 3 b show a third embodiment of an apparatus 201 for electrochemical coating, wherein a coating reservoir 202 with a charge opening 205 and a supply reservoir 203 are on one side of a common rotation axis A″, wherein, again, a counterweight 218 is coupled to reservoirs 202, 203. Here, both reservoirs 202, 203 have the same distance from rotation axis A″, but supply reservoir 203, in the rest position shown in FIG. 3 a, is below coating reservoir 202, while, in the coating position shown in FIG. 3 b, it is above coating reservoir 202. A passage opening 204 is between reservoirs 202, 203. The two reservoirs 202, 203 are rotated in such a manner that they are passed over common rotation axis A″. The apparatus 201 shown works according to the same principle as an hourglass. To ensure draining of coating liquid 220 as fast as possible, again, an opening for gas exchange 215 is provided that is arranged in such a manner that it always remains free of coating liquid 220.

The electrodes are not shown in FIGS. 2 and 3 to show the functioning principle of the coating apparatus with better clarity.

In a case where the workpieces have to be laboriously cleaned and prepared for coating, in particular, or where, after coating, rinsing processes for recovering the ionic liquid are provided, the apparatus according to the present invention can be installed in a housing or in a room in which a controlled room climate is present, e.g. having particularly low air moisture of below 1%, preferably below 0.1%. 

1. An apparatus for electrochemically coating a workpiece, comprising a coating reservoir (2, 102, 202) for receiving the workpiece (22) and a supply reservoir (3, 103, 203) for coating liquid (20, 120, 220) communicating with the coating reservoir (2, 102, 202) through at least one passage opening (4, 104, 204) for the coating liquid (20, 120, 220), a charge opening (5) for introducing the workpiece (22) into the coating reservoir (2, 102, 202), and at least one electrode (11) for contacting the workpiece (22) and at least one counter electrode (14), wherein the apparatus (1, 101, 201) is switchable between a coating position and a rest position by rotating the coating reservoir (2, 102, 202) and the supply reservoir (3, 103, 203) about at least one common rotation axis (A, A′, A″) in such a manner that the center of gravity of the volume of the coating reservoir (2, 102, 202) in the coating position is lower in relation to the center of gravity of the volume of the supply reservoir (3, 103, 203) than in the rest position.
 2. The apparatus according to claim 1, wherein, in the coating position and in the rest position, the coating reservoir (2, 102, 202) and the supply reservoir (3, 103, 203) are on the same side of the rotation axis (A, A′, A″) with reference to a vertical plane extending through the rotation axis (A, A′, A″).
 3. The apparatus according to claim 1, further comprising an opening (15, 215) for gas exchange between the coating reservoir (2, 102, 202) and the supply reservoir (3, 103, 203).
 4. The apparatus according to claim 1, further comprising a receiving device (7) arranged within the coating reservoir for the workpiece (22).
 5. The apparatus according to claim 4, wherein the receiving device (7) is rotatably supported.
 6. The apparatus according to claim 1, wherein, in the coating position, at most 50% of the volume of the supply reservoir (3, 103, 203) is below the passage opening (4, 104, 204).
 7. A method for coating a workpiece (22) in an apparatus (1, 101, 201) according to claim 1, wherein the coating reservoir (2, 102, 202) and/or the supply reservoir (3, 103, 203) are at least partially filled with a coating liquid (20, 120, 220), comprising the following steps performed in succession: a) introducing the workpiece (22) into the coating reservoir (2, 102, 202), b) electrochemically depositing a coating on the workpiece (22) while the apparatus (1, 101, 201) is in the coating position, c) switching the apparatus (1, 101, 201) into the rest position, and d) removing the workpiece (22) from the coating reservoir (2, 102, 202).
 8. The method according to claim 7, wherein a volume of coating liquid (20, 120, 220) between 80% and 110%, of the volume of the supply reservoir (3, 103, 203) is used.
 9. The method according to claim 7, wherein an ionic liquid is used as the coating liquid (20, 120, 220) comprising ions of at least one element.
 10. The apparatus according to claim 1, wherein, in the coating position and in the rest position, the coating reservoir (2, 102, 202) and the supply reservoir (3, 103, 203) are on different sides of the rotation axis (A, A′, A″) with reference to a vertical plane extending through the rotation axis (A, A′, A″).
 11. The method according to claim 7, wherein a volume of coating liquid (20, 120, 220) between 95% and 105% of the volume of the supply reservoir (3, 103, 203) is used.
 12. The method according to claim 7, wherein a volume of coating liquid (20, 120, 220) between 99% and 101% of the volume of the supply reservoir (3, 103, 203) is used.
 13. The method according to claim 9, wherein the at least one element comprises aluminum. 